Stabilized d. c. amplifier



May l0, 1960 c. E. BERRY STABILIZED n.0. AMPLIFIER Filed Aug. 13, 1956 --.l.---..- u l@ y m A u i ,l, 4 m S #u m4 u NU Y lm .m l l I I l l Il L I D ,WwW mn m y vw Mkl .n n .,u...A mw W@ W hln w U mm 5 Klv/1 QEYQQSGQ mw mbozow v N mv. mm. .o .v E Cid \w| mv ON w m uhu @W mw\\ Q l? wh. Q V NN mQ` wm. l 2)? c 0 om, umm m2 |b| .H2 m..

United States Patent O rmice s'rABILrzED nc. AMPLIFIER Clifford E. Berry, Altadena, Calif., assignor to Bell & `Howell Company, Chicago, Ill., a corporation of Illinols Application August 13, 1956, Serial No. 603,602

4 Claims. (Cl. 330-9) This invention relates generally to direct current arnplifiers, and more particularly, is concerned With a chopper-stabilized differential amplifier.

The problem of drift in D.C. amplifiers, both short term and long term drift, is well known, and various circuits `have heretofore been proposed for stabilizing the D.C. amplier to minimize these drifts. Onetechnique which has been successfully employed in single-ended amplifiers, i.e. amplifiers having both their input and output referenced to ground potential, has been the use of a chopper to alternately sample the input and output and use the sample difference to control the operating or zero point of the amplifier so that with zero input voltage, the output is zero. Where both the input and output are referenced to `ground potential, comparison of the input and output by the chopper is readily accomplished. 4

However, in many measurements applications, it is not readily possible to have one side of the input at ground potential. For eXample, the input to the amplifier may be derived across one diagonal of a bridge circuit while one end of the opposite diagonal is grounded. Thus the voltage to be measured and applied across the input to the D.-C. amplifier is floating withl respect to the ground reference potential. In such case the general Vpractice is to use a differential amplier. Since the input is floating with respect to ground, a direct comparison of the outputV to the input by means of a chopper has not been used heretofore for stabilizing a differential amplifier.

In accordance with the present invention stabilization of a D.C. differential amplifier is accomplished by means of a chopper, even though the input to the differential amplifier is iioating with respect to ground potential.

This is achieved by means of a storage capacitor whichl is` alternately connected across a iirst potential proportional to the input signal and then to a second potential proportional to the output signal. Both ends of the storage capacitor are switched by the chopper, so that the capacitor acts as a means for comparing potential dilierences regardless of the absolute potentials of the two differences being compared.

ln brief, the invention contemplates a circuit including a rst differential amplifier stage connected to the input terminals, followed by a constant gain direct-coupled. amplifier and cathode follower coupled to the output terminals. Means including a low-pass filter derives a first potential proportional to the voltage` across the input terminals, and means including an attenuator and a low-pass filter derives a second potential proportional to the voltage appearing across the output terminals. A double-throw double-pole chopper switch alternately connects the two ends of a storage capacitor across the input and the output derived potentials. The storage capacitor is coupled through a differential amplifier and an alternating current amplifier, to a synchronized demodulator circuit, which is synchronized with the chopper. The

output of the demodulator is coupled to the bias control of the first stage of the D.C. amplifier, whereby the zero,A point of the amplifier output is controlled in response to differences in the first and second potentials.

For a better understanding of the invention reference should be had to the accompanying drawing, wherein the single figure is a schematic diagram of the amplifie i circuit of the present invention.

lReferring to the figure in detail, a D.C. input signal, f

which is at a floating potential with respect to ground,

ferential amplifier.

couple the plates of the respective triodes 14 and 16 to a suitable B+ supply 21. The cathodes of thetriodes 14 and 16 are connected through a common bias resistor 22 to a B- source 23. The resistance 22 is quite Vlarge to provide a substantial constant current source, as re`- quired to provide a suitable differential amplifier.

The plate of the triode 14 is directly coupled to the input Aof a single-ended input D.C. ampliiier 24. Be-

cause the total cathode current of the differential amplifier stage is constant, changes in the average level of the input with respect to ground affect the plate voltage of the triode 14 by a negligible amount. ampli'er 24 is coupled to a pair of output terminals 26 and 28 through a cathode follower circuit including a triode 3?, the plate of which is connected to the B+` supply 2l and the .cathode of which is connected to the B-supply 23 through a load resistor 32. The output terminal 26 is connected to the cathode of the cathode follower triode 30 and the output terminal 28 is connected to a grounded reference common to the D.C.,"

amplifier 24.

In order to maintain the output voltage across the tere` minals 26 and 28 constant with a constant input voltage across the input terminals 10 and'12, regardless of drift.

in the several stages of amplification, the output voltage is connected to an attenuator 34 and low-pass lter 36` to one pair of terminals of a double-throw double-pole switch indicated generally at 38. The switch 38 connectsV the attenuated version of the output voltageacross a' storage capacitor 40 when the switch is thrown in one direction. The switch 38 is part of a synchronous chop-H per, indicatedgenerally at 42, which is driven by meansn of acoil 44 connected to an alternating current source 45.U The chopper switch 38 when thrown to its oppositeposition connects the capacitor -40 across a voltage derived4 from the input terminals 10 and 12. To this end the input terminals are connected to a low-pass lter 46 and through a pair of isolating resistors 48 and 50 to the chopper switch 38. `If the potential stored on the ca-,l

pacitor tti produced by the signal derived :from the outputl is not equal to the potential at the input, a voltage di`fference is produced across the resistors 48 and 50.

The resistors 48 and 50 are connected also to the con; trol grids of a pair of triodes 52 and 54 comprising a single stage of a differential vamplifier similar to ythe differential amplifier involving the triodes 14 and 16.` The plate of the triode 52 is coupled to the input of an A.C. amplifier 56, the output of which is fed to a syn-A chronous demodulator 58. Any conventional demodula# tor circuit, such as a chopper or ring modulator, preferably having a single-ended output, may be used. The synchronous demodulator S8 is connected to the same alternating current reference source as the chopper 42.1

Patented May 10, 1960 The output of the'y weer The output from the demodulator 58 is utilized to control the bias of the input stage in the D.-C. amplifier 24.

In operation it will be seen .that the storage capacitor 40 periodically samples the potential across Ythe output; The resulting potential stored on the capacitor is then compared with the input voltage following cach of the periodic samplings of the output. If any difference in potential exists between ythe input and the output, the potential on the grids of the triodes 52 and 54 will shift abruptly Ifrom the level set by the input to the charge level of the capacitor 40 when the capacitor 40 is connected across the grids.I The .isolation resistors 48 and 50 prevent Ithe capacitor 40 Ifrom discharging or charging to the input potential during the .time it is connected across the input. Thus a square wave is produced at the plate of thetriode 52 -having the same period as the reference alternating voltage applied to the chopper 42, theamplitude of the square wave and the phase of the square wave being indicative respectively of the difference inma-gnitude of the input andV output voltages and their relative polarity. The output of the demodulator 58 is therefore a D.-C. voltage which varies in magnitude with changes in the output voltage not produced by changes in the input voltage. The output of the demodulator controls the bias on the input stage of the amplifier 24 to either reduce or increase the output voltage, so that the outputV from the attenuator 34 is made equal to and held equal to themagm'tude of the input signal. It lwill be` seen that the amount of attenuation of the attenuator 34'which is by definition always less than unity, should be equal to the reciproca-1 of the over-all gain between the input and the output of the amplifier to make the output of the attenuator substantially equal to the input to the amplifier.

The purpose of the low-pass filter 46 in the input is to reduce signals having the chopper Ifrequency to Vsuch a, level as to prevent zero shift due .to rectification by the chopper. The low-pass lfilter 36 on the output is then necessary for proper transient response. The storage Capacitor 40 should be made sufficiently small so that it does not `disturb appreciably the low-pass filter 36 which connects it to the output. The resistors 48 and 50 are then made sufiiciently large so that the capacitor 40 does not appreciably disturb .the input filter 46.

From the above description it will be seen that a amplifier is provided having a double-ended input and yet is stabilized against drift by a chopper-controlled feedl back circuit. Thus the amplifier of the present in- `gention is stabilized against drift regardless of 4the averagepotential level of the input signal relative to ground.

l claim:

1. A stabilized direct current amplifier circuit for coupling aV pair of input terminals that are fioating with respet to a ground reference potential to a pair of output terminals, one of which is at the ground reference potential, said circuit comprising `a first differential am- .plifier stage connected toV the input terminals, la directcoupled amplifier connected .to receive the difference signal from the differential amplifier and connected to the ground reference, means coupling the output of the direct-coupled amplifier Vto the output terminals, a storage capacitor, a double-pole double-.throw chopper switch, means for deriving a tirst potential proportional to the potential appearing across the input terminals including a low-pass filter, means for deriving a second potential proportional to .the potential appearing across the output terminals including .attenuating means and a lowpass filter, the attenuating means having an attenuation substantially equal Ito the reciprocal of the gain produced by the circuit between the input and output terminals, the chopper switch alternately connecting the 'first and second potentials across the storage capacitor, a second diferential amplifier stage, means Ifor directly coupling said first potential to the second differential amplifier stage, an

alternating currentamplifier coupled to one output side.

ofthe second differential amplifier stage, a demodulatnr. synchronized with the chopper switch coupled to the output of the .alternating current amplifier, and means .for

controlling the zero point of the direct-coupled amplifier in response to the output of the demodulator.

2. A stabilized direct current amplifier circuit for coupling a pair of input terminals that are clioating with respect to a ground reference potential to a pair of output terminals, one of which is at the ground reference potent-iai, said circuit comprising a first differential amplifier stage connected to the input terminals, at least one Vdirect-coupled amplifier stage connected to receive the difference signal fromv thedifferential amplifier and connected to the ground reference, means coupling the `directcoupled amplifier stage to said output terminals, a storage capacitor, means yfor deriving a first potential proportional to the potential appearing across the input terminals, means for deriving a second potential proportional to the potential appearing across the output terminals including attenuating means having an attenuation substantially equal tothe reciprocal of the gain produced by theV circuit between .the input and output terminals, means for connecting,v the second potential across the storage capaci-A tor for charging the capacitor 4tosaid second potential, a second differential amplifier stage, means for directly coupling said first potential to the second differential amplifier stage, means lfor periodically disconnecting both ends of the storage capacitor from the output of thel means for deriving the second potential and connecting the ends of the capacitor `across ythe input to the second differential amplifier stage, an alternating current arnplifier coupled to receive the difference signal from the second differential amplifier stage, and a synchronous demodulator for coupling the output of the alternating current amplifier to the direct-coupled amplifier -for biasing the amplifier to'a fixed zero point.

3. A stabilized direct current amplifier circuit for coupling a pair of input terminals that are floating with respect to a ground reference potential to a pair of output terminals, one of whichis at the ground reference potential, said circuit comprising a first differential amplifier stage connected to the input terminals, a directcoupled amplifier stage connected to receive the difference signal from the differential amplifier and connected to the ground reference potential, the output of the direct Vcoupled amplifier stage'beingV coupled to the output terminals, 'a'storagecapaciton means for deriving a first potential proportional to the potential appearing across the input terminals, means for deriving a second potential proportional to the potential appearing across thel outputterminals, means for connecting the second potential across the storage capacitor to` charge ythe capacitor to said second potential, a second differential amplifierA stage, means` for directly coupling said first potential to the second ldifferential amplifier stage, means for periodically disconnecting both ends of the storage capacitor yfrom the output of the means for deriving the secondpotential and connecting the ends of the capacitor across the input to the second differential amplifier stage, an alternating current amplifier coupled to receive the difference signal from .the second differential amplifier stage, and a synchronous `demodulator for coupling the output of the alternating current amplifier to the direct-coupled vamplifier stage for varying the bias of said stage whereby thepz'ero point `amplifier stage is controlled in response to `differences in the first and second potentials.

4. A stabilized direct current amplifier circuit cornprisingdirect current amplifying means including at least. one stage having variable bias control, the amplifying means having a double-ended input and a single-ended output, a'storage. capacitor, means for deriving a potential rbetween a pair of terminals, in response to `the output of the lamplifying means, comparing means havingk first and second. pairs.. of input terminals for yderiving anV out- 6 put potential proportional to the difference between the plifying means in response to the output potential derived input potentials applied to the rst and second pairs of from said comparing means. input terminals thereof, means for alternately connecting the ends of the storage capacitor to the terminals of the References Cited in the flle 0f this Pl'fel-'lt means for deriving a potential from the single-ended out- 5 UNITED STATES PATENTS put of the amplifying means and to the iirst pair of input terminals of the comparing means, means -for coupling 2,709,205 Colls May 24, 1955 the second pair of terminals of the comparing means to 2,714,136 Greenwood July 26, 1955 the double-ended input of the amplifying means, and 2,724,795 Rusler Nov. 22, 1955 means for varying Ithe bias of said one stage of the am- 10 2,751,494 Gray June 19, 1956 

